1. Field of the Invention
This invention relates to a control method for obtaining a desired coiling temperature by way of cooling a rolled strip in the hot rolling process of metals and its system.
2. Related Background Art
Quality control in hot sheet metal rolling process is largely divided into two following controls: (1) Product size control such as strip thickness control for controlling rolled strip thickness in its lateral center, strip width control, strip crown control for controlling lateral width distribution, and flatness control for controlling strip lateral elongation, and (2) temperature control of rolled strip. The temperature control of rolled strip includes two following controls: (1) Temperature control for controlling the temperature of rolled strip at the delivery side of finishing rolling mill and (2) coiling temperature control for controlling the temperature of the rolled strip in front of the coiler.
Generally, in a hot rolling mill, a heating furnace, a roughing mill, a finishing mill, a run out table (ROT) on which a cooler is installed and a coiler are serially arranged. Typical temperatures of strips are: 1200 to 1250 degree C. at the delivery side of the heating furnace, 1100 to 1150 degree C. at the delivery side of the roughing mill, 1050 to 1100 degree C. at the entry side of the finishing mill, 850 to 900 degree C. at the delivery side of the finishing mill, and 500 to 800 degree C. at the coiler. In almost all cases, the strength, toughness and other properties of rolled strips depend on positive cooling to which the strips are subjected while the strips come out from the finishing roll and reach the coiler. Therefore, coiling temperature control is extremely critical for final material quality.
FIG. 10 is a schematic block diagram showing a typical coiling temperature controlling system according to the prior art, inclusive of applications: In the drawing, after a stripping sheet 1 is finishing rolled into the strip 1 at the finishing mill 4, the strip 1 is transported on the ROT and finally coiled by the coiler 6 while guided by the pinch roll 5. The finisher delivery pyrometer (FDT) 2 is provided at the delivery side of the finishing mill 4, and the coiling pyrometer (CT) 3 is provided at the entry side of the pinch roll 5. On the ROT 10 is installed a cooling device consisting of n pieces of cooling units (also collectively referred to as cooling bank) 7a, 7b, 7c, . . . . The cooling units respectively inject cooling water to cool the strip 1. In this connection, in the drawing ROT 10 is drawn like a straight line, but actually a number of rolls are arranged for rotation, to transport the strip 1.
The valves installed in the cooling banks 7a, 7b, 7c, . . . for controlling cooling water flow rate may be closing valves or flow control valves. But, the two or three cooling banks nearest to the coiling pyrometer 3 may be flow rate controllable valves or a number of small flow rate closing valves to have finer feedback control, which is to be described in more detail later.
The coiling temperature controller 24 is installed to control the opening and closing of each valve at the cooling banks 7a, 7b, 7c, . . . to control cooling water flow rate. To the coiling temperature controller 24 are fetched the temperature indications of the finishing delivery pyrometer (FDT) 2 and the coiling pyrometer (CT) 3, output pulses of the pulse generator 9a connected the driving motor of the finishing roll 4 and the pulse generator 9b connected the coiler 6, as well as calculational information for setting the finishing roll 4 which is made by the finishing roll setting calculation means 8.
The coiling temperature (CT) control system 24 is divided into the following two subsystems from the viewpoint of its purpose: (1) The first subsystem which determines which cooling banks 7a, 7b, 7c, . . . is or are used for cooling so that the CT should coincide with the target coiling temperature T.sub.CT.sup.AIM, mainly based on the temperature measurement T.sub.FD.sup.ACT of the strip 1 (detected by FDT 2) locating right thereunder, and (2) the second subsystem which corrects a deviation of actual coiling temperature T.sub.CT.sup.ACT from the target coiling temperature T.sub.CT.sup.AIM.
The first subsystem consists of the material temperature prediction means 13, the material tracking means 14, the cooling water flow rate setting means 22 and the temperature model learning means 23, while the second subsystem consists of the target temperature correction means 16, the feedback control means 17 and the cooling water flow rate changing means 21.
Now, description will be made for the CT control system 24 according to the prior art as follows:
According to the prior art, a total length of rolled strip 1 is divided into a number of conceptual segments as a material cooling unit. The performance of the cooling banks 7a, 7b, 7c, . . . are decided, so that, at the point of time when a certain segment of the rolled strip passes a specific rolling stand (e.g., the (m-j)th stand) of the finishing rolling mill 4, the segment temperature should become the target coiling temperature T.sub.CT.sup.AIM, which is calculated based on the temperature measurement T.sub.FD.sup.ACT of the strip 1 locating right under FDT 2 and the setting calculational information of the rolling mill setting calculation means 8. Therefore, by counting the output pulses of the pulse generators 9a and 9b, the material tracking means 14 detects the location of the strip 1 on ROT 10 at any state at the time of (1) "before the head end of the strip 1 reaches the coiler 6", (2) "while coiling the strip 1", and (3) "after the tail end of the strip 1 passes through the finishing mill 4".
In this connection, tracking of the strip 1 is not limited to the method which counts the output pulses of the pulse generators 9a and 9b, but, for example, another method such as provision of material sensor midway of ROT 10 can be used.
The temperature model learning means 23 provides necessary information for prediction of material temperature to the material temperature prediction means 13, based on the temperature measurement T.sub.FD.sup.ACT of the strip detected by FDT 2 and the actual coiling temperature T.sub.CT.sup.ACT to be detected by CT 3.
At a timing when the k-th strip segment from the head end of the strip 1 just reached the (m-j)th stand of the finishing mill 4, the material temperature prediction means 13 predicts the probable material temperature which takes place when the k-th segment is to be applied with cooling water at the cooling bank 7a. The cooling water flow rate setting means 22 judges whether the material temperature predicted at the (m-j)th stand can achieve the target coiling temperature T.sub.CT.sup.AIM. When "YES", only the cooling bank 7a is used. When "NO" or higher than target, the downstream cooling bank 7b is used together. Then, again, the material temperature is estimated by the material temperature prediction means 13. The above-described operation is repeated until the target coiling temperature T.sub.CT.sup.AIM is obtained.
In this connection, why these calculations must be done at the timing when the segment just reached the (m-j)th stand of the finishing mill 4 is as follows: In general, an opening or closing of the valves or a flow rate change to be made in the cooling bank controller would have dead time or response delay, or the calculational operation to be done therein would take much time, thereby necessitating the compensation for these delays. Therefore, when these loss times can be minimized, a referenced stand such as the (m-j)th stand can be brought to more downstream stand, expecting more accurate coiling temperature.
Thus, when the cooling water flow rate to be applied to a reference segment is determined, and when the referenced segment reached the i-th stand while the material tracking means 14 was keeping the track of the segment, the desired cooling water flow can be supplied.
Then, when the cooled k-th segment reached just under the CT 3, the feedback control means 17 determines a deviation of T.sub.CT.sup.ACT from T.sub.CT.sup.AIM, and adjusts the flow rates at e.g., "(n-1)"th and "n"th cooling banks so as to minimize the deviation.
When the deviation of T.sub.CT.sup.ACT from T.sub.CT.sup.AIM is significant, the target temperature correction means 16 provisionally changes the target temperature. For example, when the measured T.sub.CT.sup.ACT is higher than T.sub.CT.sup.AIM, the target temperature is purposefully lowered for a while. A valve opening triggered by the cooling water flow rate changing means 21 for following the lower target temperature can accelerate the sooner approach of T.sub.CT.sup.ACT to the original target temperature T.sub.CT.sup.AIM.
Before the rolled strip enters ROT 10, the coiling temperature control system according to the prior art determines how much and when the cooling water should be supplied to the segments. After the determination, if there should happen a change in the temperature or transfer speed of a segment at the delivery side of the finishing mill (that is at the entry side of the ROT) or a large disturbance, the controllability may deteriorate significantly. Preventive actions against such deterioration of coiling temperature controllability are known as e.g., "Hot Rolling Mill Coiling Temperature Control" specified in JP 08090036 A and "Temperature Control Method for Hot Rolled Strip" specified in JP 10005845 A.
Among the two, the former intends to control a temperature change of the strip at the delivery side of the finishing rolling mill and a change in coiling temperature due to a change in transfer speed of the strip separately, while the latter determines the mean value of the preset speed pattern and a changed speed pattern when the strip transfer speed is changed, to recalculate the necessary cooling water flow rate. Such being the case, the coiling temperature control method according to the prior art is very insensible to an unexpected speed or a change in the entry side temperature, thus resulting in a failure to cover such insensibility.
On the other hand, in the temperature model, the locations on the ROT 10 at which strip or segment temperature can be measured are limited to e.g., the delivery side of the finishing rolling mill 4, the entry side of the coiler 6 and rarely midway of the ROT 10, so that it is qualitatively known that the lower the temperature, the higher the cooling effect. With respect to the learning of temperature model, however, it can have only one or a few learning terms at the full length of the ROT 10, thus resulting in a failure to learn the cooling characteristics at the upstream and downstream sides separately.
In a case of thick strips, the surfaces may be fully cooled, but the inside cannot be sufficiently cooled, thus causing a higher mean temperature in the thickness direction. To our regret, the surface temperature is only one measurable by way of pyrometers, so that temperature calculations based on a model cannot show good agreement with actual temperatures to be used in learning course, thus resulting in a poor temperature prediction accuracy. Further, even in the case where a mean temperature in the thickness direction is calculated, a difference equation is solved by repetitive calculations, so that sometimes the load on the computer became significantly large.